Dynamic microphone



Sept. 20, 1955 H. F. OLSON ET AL DYNAMIC MICROPHONE Filed Dec. 29, 1950 o 7 37! 3x25 f5/ 23. my/wl/y f3/A9, f/ nal/I I. 9 L Smm www /W u 6. MM5 H 5. .1 f Nn f.. F @E MRM m@ 9m, m z .2 7 @un W l f 4. a 7J 5 57 m .1.5% v/ j 4. F F Li; ///f/2\\\\.. \\n= \|.l J g. 7 a4 /7/7/ i f 5 f 4. ga 7 L f ATTO RNEY DYNAMIC MICROPHONE Harry F. Olson, Princeton, and John Preston, Metedeconk, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application December 29, 1950, Serial N o. 203,302

3 Claims. (Cl. 181-31) This invention relates to sound translating apparatus, and more particularly to an improved pressure responsive, dynamic microphone.

A pressure responsive, dynamic microphone consists of a diaphragm coupled to a voice coil located in a magnetic eld, and suitable damping means for controlling the motion of the diaphragm. The motion of the coil in the magnetic field leads to the induction of a voltage in the coil which is proportional to the velocity of the voice coil. The voltage induced in the coil is then conveyed to suitable amplifying apparatus for further use in various manners well known in the art.

The general trend in the sound translating apparatus field, insofar as microphones are concerned, is in the direction of small units for the intimate type of pickup. Consequently, there is felt the need for a small, relatively inconspicuous, pressure responsive, dynamic microphone.

lt is, therefore, a primary object of the present invention to provide a small, relatively inconspicuous, pressure responsive, dynamic microphone.

lt is also an object of the present invention to provide a pressure responsive, dynamic microphone which will have an improved response characteristic in the high frequency range.

A further object of the present invention is to provide a dynamic type microphone having excellent sound reproducing qualities and which is relatively inconspicuous to view.

Another object of the present invention is to provide a pressure responsive microphone which is so compact in structure that it is easily portable.

Still another object of the present invention is to provide a pressure responsive, dynamic microphone which is simple to construct, inexpensive in cost and highly eflicient in use.

In general, a microphone constructed in accordance with the present invention comprises a case, a sound responsive diaphragm disposed across an opening leading into the case, and an acoustic resonating chamber mounted on the case above the diaphragm. In order to accentuate high frequency response, the effective length of the chamber is made comparable to a wavelength at the lower limit of the frequency range in which maximum effect is desired.

The novel features characteristic of the present invention, as well as additional objects and advantages thereof, Will be better understood from the following detailed description when read in connection with the accompanying drawing, in which:

Figure 1 is a front elevation of a microphone in accordance with the present invention,

Figure 2 is an enlarged, central longitudinal section of the microphone shown in Figure 1,

Figure 3 is an enlarged view of the sound translating mechanism shown in Figure 2, upper and lower portions of the microphone casing being broken away,

nited States Patent O ice Figure 4 is a plan view of the microphone shown in Figure 1,

Figure 5 shows two curves one of which indicates the response characteristic of a microphone with a resonator in accordance with the present invention, and the other of which indicates the response characteristic without the resonator, and

Figure 6 is a view similar to Figure 3 of another embodiment of a sound translating mechanism in accordance with the present invention.

Referring more particularly to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown a microphone 1 of the dynamic, pressure responsive type. The microphone 1 comprises a sound translating mechanism 3 and a coupling transformer 5 mounted in a lightweight housing or case 7.

The sound translating mechanism 3 is of the dynamic type employing a conventional center magnet system. The structure of the magnet system is generally cylindrical in shape and includes a field yoke 9 of permeable magnetic material, an annular, outer pole plate 11 of permeable magnetic material, a substantially cylindrical inner pole member 13 of permeable magnetic material, and a frusto-conical permanent magnet 15 connecting the inner pole member and the iield yoke. The outer pole plate has a central, circular aperture 17 within which the inner pole member 13 is concentrically disposed in spaced relation to the outer pole plate so as to provide an annular air gap 19.

A diaphragm 21 is mounted for vibratory movementon the outer pole plate 11, being responsive to sound wave pressure impinging on the surface thereof. The diaphragm 21 is of a conventional dome-shape type having a voice coil 23 attached at the base of the dome 25 and supported within the air gap 19. An annular compliance 27 surrounds the dome having its inner edge attached to the dome and its outer peripheral portion clamped between two rings or washers 29. The annular compliance 27 may be of any type found suitable for obtaining a compliant suspension which will not break up into nodes in the high frequency range. It may be noted that a suspension of the fluted type has been found suitable.

The diaphragm 21 is made from material suitable for obtaining a low fundamental resonant frequency and which will withstand any direct force duc to Wind or shock. Aluminum was not found to be suitable for such a diaphragm since it was impossible to obtain a sufficiently low fundamental resonant frequency without using exceedingly thin material which was so fragile that it would not withstand any direct force of the type above mentioned. Best results were obtained With a Vinylite diaphragm and suspension system having a thickness of the order of .0035 inch. Vinylite is a synthetic resin which is a copolymer of vinyl chloride and vinyl acetate and containing about to about 9() percent by weight polyvinyl chloride. A diaphragm and suspension of this construction is found to be very rugged and will not collapse or become permanently injured by shock or large steady forces. However, the Vinylite diaphragm was found not to support high frequency vibrations since it was not rigid enough. Therefore, in order to improve the rigidity of the diaphragm and extend the high frequency responses thereof, a paper dome 30 was laminated to the dome of Vinylite.

The back of the diaphragm is terminated in a mechanical resistance comprising a ring of felt 31 disposed around the inner pole member 13 between the outer pole plate 11 and another annular ring of non-magnetic material 32 which also surrounds the inner pole member.

The lightweight housing or case 7 of the microphone comprises a cylindrical shell having one end 33 open.

The sound translating mechanism 3 is mounted in the upper portion of the case 7 so that the diaphragm 21 closes the open end and seals off the interior of the case from the ambient. The coupling transformer 5 is mounted within a casing 34 which provides a shield therefor. The transformer S and its case 34 are mounted in the lower portion of the shell. Leads 35 connecting the voice coil 23 with the primary coil of the transformer 5 extend through the hollow central portion of the shell. In order to prevent undesirable resonances in the hollow central portion, cotton or other suitable damping material (not shown) may be used to fill the space.

In order to accentuate the response of the microphone in the high frequency range, a resonator 37 is provided. The resonator 37 comprises a walled enclosure defining an acoustic cavity or chamber of truncate conical form. The base of the chamber is mounted over the open end 33 of the housing 7 such that the side of the diaphragm 2,1 which is exposed to the ambient is within the resonator.

The base of the chamber is of a diameter substantially f equal to the diameter of the diaphragm 21, that is, it provides an opening at least as large as said diaphragm. The walls of the resonator are provided with a plurality of elongated, substantially triangular shaped slots or sound transmitting passages 39 through which sound waves are transmitted to the acoustic chamber and the diaphragm 21. A screen 41 of suitable material lines the interior of the resonator to cover the slots and function as a wind and dust screen. The resonator 37 may be formed as an integral part of the housing '7 or it may comprise a separate member attached to and extending from the end of the housing containing the sound translating mechanism 3. ln either case, the triangular shaped slots 39 should be arranged with the sides forming the bases disposed substantially in a plane including the diaphragm 21 thereby to prevent undesirable resonances which might otherwise result from a cavity disposed in front of the diaphragm. ln addition thereto, it is found that, in order to be effective to accentuate response in the high frequency range, the length of the acoustic chamber should be comparable to a Wavelength at the lower limit of the frequency range in which maximum effect is desired and the slots should be at least as long as the acoustic chamber. Also the bases of the slots 39 should be of a length approximately one-half wavelength at the upper limit of the frequency range in which maximum efect is desired. For example, if the lower limit of the frequency range in which accentuation in response is desired is approximately 2000 cycles per second, the length of the acoustic chamber should be at least 6 to 7 inches. lf the upper limit of the frequency range in which accentuation in response is desired is approximately 12,000 cycles per second, the length of the slot bases should be approximately (ym of an inch.

The effect upon the high frequency response introduced by the resonator .37 of the present invention is illustrated in Figure 5. The response characteristic for the microphone without the conical resonator is represented by the solid line curve A and the response with the conical resonator is shown by the broken line curve B. lt will be seen from a comparison of these two curves that the response is accentuated over the frequency range from 2000 to 12,000 cycles per second.

Although the microphone 1 illustrated particularly in Figures 2 and 3 comprises a preferred embodiment of the present invention since it utilizes a magnetic field structure employing a permanent magnet as its source of magnetic energy, Figure 6 illustrates the application of an electromagnet 43 for supplying the magnetic energy. The microphone structure of this second embodiment is practically the same as the preferred embodiment except that, in place of a permanent magnet for supplying magnetic ux, an iron core 45 and field coil 47 are disposed between the inner pole member 13 and the iield yoke 9 and provision is made for supplying electric energy to the field coil.

From the foregoing description, it will be apparent that the present invention provides an eicient microphone which is particularly adaptable to miniature construction thereby to make it relatively inconspicuous. Although there areY shown and described but two embodiments of the present invention, it will be apparent to those persons skilled in the art that other modifications and changes are possible within the spirit of the present invention. For example, a different arrangement may be provided for terminating the rear side of the diaphragm with an acoustic resistance. Also other mounting means may be provided as a support when the microphone is not held by the hand of a user. Other changes of like character will, of course, readily suggest themselves. Therefore, it is desired that the particular forms of the present invention shown and described herein shall be considered as illustrative and not as limiting.

What is claimed is:

1. An acoustic device having a response characteristic extending throughout a range of frequencies and having an accentuated response at the higher end of said range comprising a case having an open end, a diaphragm in said case closing ysaid open end and mounted for movement in response to sound waves impinging thereon, an acoustic resonating chamber in the form of a truncated cone, said resonating chamber comprising a walled enclosure having elongated openings therein through which sound waves are transmitted to said acoustic chamber, and said acoustic chamber being mounted on said case at the side of said diaphragm opposite said case and in axial alignment with said case, said acoustic chamber having an effective length substantially equal to the wavelength which corresponds to the lower limit of the higher frequency range of the device whereby to accentuate the response of said device in the frequency range lying above said lower limit.

2. An acoustic device according to claim 1 wherein said elongated openings are substantially triangular in shape and are at least as long as said acoustic chamber, and wherein the bases of said openings are disposed substantially in a plane including said diaphragm.

3. The invention as defined in claim 2 wherein the bases of said elongated openings are of a length approximately equal to one-half wavelength at the upper limit of the frequency range in which maximum aecentuating effect is desired, and wherein the length of said slots is substantially the same as the effective length of said chamber.

References Cited in the file of this patent UNITED STATES PATENTS 1,778,693 Semple Oct. 14, 1930 1,912,454 Hutter June 6, 1933 1,917,309 Leopold July 11, 1933 1,939,447 Honig Dec. 12, 1933 1,960,449 Midgley May 29, 1934 2,217,279 Karns Oct. 8, 1940 2,228,886 Olson June 14, 1941 2,271,988 Olson Feb. 3, 1942 2,277,525 Mercurius Mar. 24, 1942 2,566,094 Olson Aug. 28, 1951 FOREIGN PATENTS 485,239 France Dec. 20, 1917 22,086 Australia Apr. 23, 1936 

